1. Field of the Invention
The present invention relates to an AC-type plasma display panel and method for driving the same intended to reduce power dissipation and black luminance.
The present application claims priority of Japanese Patent Application No. 2001-356926 filed on Nov. 22, 2001, which is hereby incorporated by reference.
2. Description of the Related Art
Generally, a plasma display panel (hereinafter may be referred to as PDP) has many features such as thinness, comparatively ease to provide large-screen display, a large visibility angle, and a high response speed. Due to these features, a PDP has recently been utilized as a flat display in a wall-mounted TV, a public display board, or a like. The PDPs are classified by an operation type thereof into a Direct Current discharge-type (DC type) PDP in which electrodes are arranged, as exposed, in a discharge space filled with a discharge gas to generate DC discharge between the electrodes in operation and an Alternating Current discharge-type (AC type) PDP in which electrodes are coated with a dielectric layer not to be exposed to a discharge gas directly in operation of the PDP in an AC discharge condition. In a DC-type PDP, discharge is sustained for a period for which voltage is being applied, while in an AC-type PDP, discharge is sustained by alternating the polarity of voltage applied. Furthermore, the AC-type PDPs are subdivided in construction into those having two electrodes in each display cell and those having three electrodes in each display cell. Such three-electrode construction of PDPs is described, for example, in “Society for Information Display '98 Digest, pp. 279-281, May, 1998”.
The following will describe a construction of a conventional three-electrode, AC-type plasma display panel and method for driving the same.
As shown in FIG. 7, this conventional three-electrode, AC-type plasma display panel is provided with a front face substrate 20 and a rear face substrate 21 arranged opposite to this front face panel 20. The front face panel 20 and the rear face panel 21 are made of glass, for example. On such a surface of the front face substrate 20 as to face the rear face substrate 21 are there arranged a plurality of scanning electrodes 22 and a plurality of common electrodes 23 alternating with each other with a predetermined spacing therebetween. The scanning electrodes 22 and the common electrodes 23 extend in a direction from a surface in FIG. 7 toward you. The scanning electrodes 22 and the common electrodes 23 are a transparent electrode made of an ITO (Indium Tin Oxide) or a like. Furthermore, on each of the scanning electrodes 22 and the common electrodes 23 is there stacked a metal electrode 32 to reduce wiring resistance. Moreover, to cover the scanning electrodes 22 and the common electrodes 23 is there provided a transparent dielectric layer 24, on which is in turn formed a MgO (Magnesium Oxide) film 25.
On such a surface of the rear face substrate 21 as to face the front face substrate 20, on the other hand, is there provided a plurality of data electrodes 29, which extends in a direction (vertical direction shown in the figure) perpendicular to the scanning electrodes 22 and the common electrodes 23. On the data electrodes 29 are there provided a white dielectric layer 28 and a phosphor layer 27.
Furthermore, between the front face substrate 20 and the rear face substrate 21 is there arranged a partition (not shown). This partition serves to preserve a space between the front face substrate 20 and rear face substrate 21 as a discharge space 26 and also divide the discharge space 26 into a plurality of display cells 31 (picture cells). The display cells each have one nearest portion between the scanning electrode 22 and the data electrode 29 and one nearest portion between the common electrode 23 and the data electrode 29. The discharge space 26 contains a mixture gas of He, Ne, Xe, or a like as a discharge gas.
Furthermore, as shown in FIG. 8, on a display screen 30 of the PDP are there arranged the display cells 31 in a matrix so that each of them may have one nearest portion between each of the scanning electrode 22 (Si (i=1−m)) and the common electrode 23 (Ci (i=1−m)) and the data electrode 29 (Dj (j=1−n)). A spacing between the scanning electrode Si and the common electrode Ci provides a surface discharge gap 33 where surface discharge occurs, while a spacing between the scanning electrode Si and the common electrode Ci−1 provides a non-discharge gap 34 where surface discharge does not occur. Note here that in this conventional PDP, the surface discharge gap 33, that is, a distance between the scanning electrode 22 and the common electrode 23 is, for example, about 70 μm, while an opposed discharge gap, that is, a distance between the scanning electrode 22 and the data electrode 29 and that between the common electrode 23 and the data electrode 29 is, for example, 120 μm. Under these conditions, surface discharge starts at a voltage of, for example, about 180V, while opposed discharge starts at a voltage of, for example, about 190V.
The following will describe a method for driving this conventional PDP. Conventionally, a method mainly used to drive a PDP has been a scanning-maintenance separated method (ADS method), whereby a scanning period and a sustaining period are separated from each other. This scanning-maintenance separated method for driving is explained below. In FIGS. 10A to 10E, a positive wall charge 35 and a negative wall charge 36 are shown in a polygon, while a height of the positive and negative wall charges 35 and 36 indicates a magnitude of a wall voltage generated by the respective wall charges on a surface of a dielectric layer. Furthermore, in FIGS. 10A to 10E, a reference symbol S indicates the scanning electrode 22 (see FIG. 7), a reference symbol C indicates the common electrode 23 (see FIG. 7), and a reference symbol D indicates the data electrode 29 (see FIG. 7). Note here that in FIGS. 10A to 10E, the MgO film 25 and the phosphor layer 27 are not shown.
In a PDP, discharge occurs in a display cell when a discharge gas is electrolytically dissociated into a positive ion and an electron and then they move in the display cell. As time passes, these positive ion and electron are recombined with each other to recover a neutral discharge gas. Therefore, the positive ion and the electron in the display cell decrease in amount as time passes. The MgO film 25 shown in FIG. 7 has a function to protect the transparent dielectric layer 24 and a function to emit a secondary electron when a positive ion in the discharge gas collies therewith. This secondary electron moves toward a positive polarity side by an electric field applied to the display cell, to collide with a molecule in the discharge gas, thus electrolytically dissociating this discharge gas molecule into a positive ion and an electron. Accordingly, more positive ions and electrons are supplied into the display cell to sustain discharge. Therefore, when starting discharge, the MgO film 25 needs to be formed on a negative polarity side beforehand always. If the MgO film 25 is not formed on the negative polarity side, no positive ion in the discharge gas collides with the MgO film and, therefore, no electron is supplied into the display cell. As a result, even if an electric field is applied to the display cell, discharge stops when positive ions and electrons present in the discharge gas before starting of discharge have all moved, so that discharge cannot be sustained.
The phosphor layer 27 shown in FIG. 7, on the other hand, emits light when irradiated with ultraviolet ray generated by discharge. An MgO film, however, does not transmit ultraviolet ray therethrough, so that the MgO film 25 cannot be formed on the phosphor layer 27. The MgO film 25, therefore, must be formed on a surface of the front face substrate 20, that is, on the scanning electrode 22 and the common electrode 23. Accordingly, to generate opposed discharge between the scanning electrode 22 or the common electrode 23 and the data electrode 29 in the display cell, the scanning electrode 22 or the common electrode 23 needs to be of a negative polarity always. Note here that to generate surface discharge between the scanning electrode 22 and the common electrode 23, either of them may be of a negative polarity.
As shown in FIG. 9, by the conventional PDP driving method, each field is made up of a plurality of sub-fields, a sub-field 8 of which is made up of three periods of a preliminary discharge period 7, a scanning period 5, and a sustaining period 6. Furthermore, the preliminary discharge period 7 is made up of a sustaining erasing period 2, a priming period 3, and a priming erasing period 4. Note here that, as shown in FIG. 9, over the entire sub-field 8, PDP driving waveforms, that is, waveforms of voltages applied to the scanning electrode 22, the common electrode 23, and the data electrode 29 are all made up of positive polarity pulses. This is because circuit costs can be reduced by thus making up the driving waveform of positive polarity pulses.
First, the preliminary discharge period 7 is explained as follows. In a previous sub-field 1 immediately preceding the sub-field 8, in each of the display cells, to the scanning electrode S is applied a positive potential Vs, while the common electrode C and the data electrode D are biased to the ground potential, The condition of a wall charge in a display cell at the beginning of the preliminary discharge period 7 depends on whether this display cell has been lit up or not in the previous sub-field 1. When discharge occurs in a display cell, an electric field in the display cell becomes uniform. Therefore, in any display cell which has been lit up in the previous sub-field 1, that is, in a display cell where sustaining discharge has been generated, an electric field in the display cell becomes uniform due to occurrence of discharge, so that as shown in FIG. 10A, the negative wall charge 36 builds up in such a region on a surface of the transparent dielectric layer 24 as to correspond to that over the scanning electrode S (hereinafter may be simply referred to as “the wall charge building up over the scanning electrode S”), the positive wall charge 35 builds up in such a region on the surface of the transparent dielectric layer 24 as to correspond to that over the common electrode C (hereinafter may be simply referred to as “the wall charge building up over the common electrode C”), and the positive wall charge 35 builds up also in such a region on the surface of the white dielectric layer 28 as to correspond to that over the data electrode D (hereinafter may be simply referred to as “the wall charge building up over the data electrode D”).
In a display cell where sustaining discharge has not occurred in the previous sub-field 1, on the other hand, as shown in FIG. 10B, the negative wall charge 36 builds up over the scanning electrode S, the positive wall charge 35 builds up over the common electrode C, and the positive wall charge 35 builds up also over the data electrode D, so that an amount of the wall charges decreases continuously both in such a region over the scanning electrode S as to be near that over the common electrode C and in such a region over the common electrode C as to be near that over the scanning electrode S (hereinafter written “near surface discharge gap” also). Therefore, a total amount of the wall charges formed in the display cell is smaller than that in a display cell in which sustaining discharge has occurred in the previous sub-field 1 (see FIG. 10A).
In the sustaining erasing period 2, the potential of the scanning electrode S is continuously decreased from the positive potential Vs to the ground potential, Furthermore, the potential of the common electrode C is fixed to the positive potential Vs and that of the data electrode D is fixed to the ground potential. Accordingly, the common electrode C is of a positive polarity and the scanning electrode S, of a negative polarity. Therefore, in a display cell where sustaining discharge has occurred in the previous sub-field 1 to form a wall charge, a wall voltage is superimposed on a potential difference between the common electrode C and the scanning electrode S, so that discharge occurs at a gap (hereinafter called inter-face gap also) between the scanning electrode S and the common electrode C. However, the potential difference between the scanning electrode S and the common electrode C increases gradually, so that strong discharge does not occur abruptly but weak discharge (feeble discharge) occurs continuously. Note here that feeble discharge refers to weak discharge which is sustained while a voltage at the discharge gap is sustained at roughly a discharge starting voltage. Accordingly, as shown in FIG. 10B, it is possible to decrease an amount of a wall charge near the surface discharge gap of those wall charges formed over the scanning electrode S and the common electrode C. Note here that in the preliminary discharge period 7, the data electrode is always biased to the ground potential.
In a display cell where no sustaining discharge has occurred in the previous sub-field 1, on the other hand, a wall charge is formed only a little in the display cell, so that inter-face feeble discharge does not occur. Accordingly, the condition of the wall charge remains unchanged in a condition shown in FIG. 10B.
Thus, by generating discharge in a display cell where sustaining discharge has occurred in the previous sub-field 1, it is possible to provide, in the sustaining erasing period 2, the same wall charge arrangement as that of a display cell wherein sustaining discharge has not occurred in the previous sub-field 1. That is, at the end of the sustaining erasing period 2, there is given such a wall charge arrangement as shown in FIG. 10B independently of whether a relevant display cell has been lit up or not in the previous sub-field 1. That is, the arrangement of a display cell wall charge can be initialized.
In the priming period 3, priming discharge is generated to obtain a priming effect in order to generate write-in discharge at a low voltage in a following process. Priming discharge occurs in each sub-field independently of whether a relevant display cell has been lit up or not in the previous sub-field 1. Therefore, priming discharge needs to be feeble in order to avoid a rise in luminance in the case of black display, that is, black luminance. As shown in FIG. 9, in the priming period 3, the potential of the scanning electrode S is increased to the potential Vs and then continuously increased from the potential Vs to a potential Vp higher than the potential Vs. That is, a voltage of a positive-polarity ramp waveform is applied to the scanning electrode S. The potential of the common electrode C, on the other hand, is set to the ground. Accordingly, the scanning electrode S becomes of a positive polarity and the common electrode C becomes of a negative polarity, so that a potential difference larger than a surface-firing voltage is applied to a gap (inter-face gap) between the scanning electrode S and the common electrode C, thus generating feeble discharge at the inter-face gap. This feeble discharge is called priming discharge. Priming discharge causes a discharge gas in a display cell to be electrolytically dissociated, thus supplying a positive ion and an electron into the display cell. Accordingly, discharge is liable to occur in the later-described scanning period 5 and sustaining discharge period 6. Note here that, when priming discharge has occurred, resultantly such a wall charge arrangement is given as shown in FIG. 10C, in which a negative wall charge builds up over the scanning electrode S, a positive wall charge builds up over the common electrode C, and a positive wall charge builds up over the data electrode D, thus providing such a condition that an amount of wall charges formed in a region near the discharge gap over the scanning electrode S and the common electrode C is larger than that formed in the other regions.
In the priming erasing period 4, the potential of the scanning electrode S is non-continuously decreased to the potential Vs and then decreased from the potential Vs to the ground potential continuously. The potential of the common electrode C, on the other hand, is set to the potential Vs. Accordingly, opposite to a condition in the priming period 3 described above, the scanning electrode S becomes of a negative polarity and the common electrode C becomes of a positive polarity. Therefore, the inter-face gap encounters feeble discharge opposite to the priming discharge described above, that is, priming erasing discharge, so that a wall charge formed by priming discharge can be erased. To prevent black luminance from rising, priming erasing discharge also needs to be feeble as in the case of priming discharge. When priming erasing discharge has occurred, resultantly such a wall charge arrangement is provided in a display cell as shown in FIG. 10D, The wall charge arrangement shown in FIG. 10D is the same as that shown in FIG. 10B, that is, a wall charge arrangement before priming discharge. Then, the preliminary discharge period 7 ends.
In the scanning period 5, a positive potential Vbw is applied to the scanning electrode S and a positive potential Vsw is applied to the common electrode C. The potential Vbw is, for example, 50 to 100V approximately and the potential Vsw is, for example, 170 to 190V approximately. In such a state, a negative scanning pulse 9 is applied to the scanning electrodes Sl through Sm. In synchronization with this scanning pulse 9 in timing, a data pulse 10 is selectively applied to data electrodes D1-Dn based on display data. The voltage of the data pulse 10 is set to, for example, 60 to 70V. In a picture cell to which the data pulse 10 has been applied, a total voltage of the scanning pulse 9 and the data pulse 10 is applied to a gap (hereinafter called opposed gap) between the scanning electrode S and the data electrode D. Accordingly, a potential difference across an opposed gap exceeds an opposed-firing voltage, thus generating write-in discharge. Furthermore, since the positive potential Vsw is applied to the common electrode C, when the write-in discharge described above occurs, correspondingly a charge moves in the gap (inter-face gap) between the scanning electrode S and the common electrode C.
Note here that, as described above, to generate opposed discharge, it is necessary to set a polarity of the scanning electrode S to be negative with respect to that of the data electrode D. Furthermore, in the present PDP, waveforms all need to be of a positive polarity in order to reduce the circuit costs. Therefore, by biasing the scanning electrode S to the ground potential in a pulse shape with respect to the potential Vbw, the scanning pulse 9 of a negative polarity is realized.
Furthermore, in write-in discharge, the scanning electrode S is of a negative polarity and the data electrode D, of a positive polarity. Accordingly, to generate write-in discharge efficiently, before write-in discharge occurs, it is necessary to have a negative wall charge over the scanning electrode S and a positive wall charge over the data electrode D beforehand. When write-in discharge occurs in such a state, the wall charge over the scanning electrode S turns positive. At this moment, the wall charge over the common electrode C must be of a negative polarity already in order to generate sustaining discharge in the following sustaining period 6. As shown in FIG. 10B, however, at the end of the sustaining erasing period 2, a negative wall charge is already formed over the common electrode C to thus permit only feeble discharge to occur in the priming period 3 and the priming erasing period 4, so that even at the end of the priming erasing period 4, the polarity of the wall charge over the common electrode C is negative. As described above, when generating write-in discharge, therefore, it is necessary to apply a positive potential to the common electrode C to thereby generate surface discharge in write-in discharge, thus reversing the polarity of the wall charge over the common electrode C.
As a result, in a display cell where write-in discharge has occurred, the scanning electrode S is of a negative polarity as shown in FIG. 10E, so that a positive wall charge builds up over the scanning electrode S. Furthermore, since the common electrode C is biased to a positive polarity potential, a negative wall charge builds up over the common electrode C. Furthermore, the data electrode D is positive in polarity with respect to the scanning electrode S but negative with respect to the common electrode C, so that little wall charge builds up over the data electrode D.
In a display cell where the data pulse 10 is not applied, on the other hand, application of the scanning pulse 9 alone is not enough to permit the potential of the opposed gap to reach the opposed-firing voltage, so that write-in discharge does not occur. Accordingly, the condition of the wall charge remains unchanged. Thus, two conditions of a wall charge can be created for each display cell by applying or not allying the data pulse 10. In a hatched portion of the data pulse 10 in FIG. 9, the data pulse 10 may be applied or not applied according to display data.
When the scanning pulse 9 has been applied to all of the scanning electrodes S (Sl-Sm), the sustaining period 6 is entered. In the sustaining period 6, a sustaining pulse is applied to all of the scanning electrodes S and all of the common electrodes C alternately. The voltage Vs of the sustaining pulse is supposed to be of such a value that surface discharge may occur in a display cell where write-in discharge has occurred in the above-mentioned scanning period 5 to form a wall charge as shown in FIG. 10E but may not occur in a display cell where write-in discharge has not occurred and so a wall charge arrangement may remain unchanged as shown in FIG. 10D. The voltage Vs of the sustaining pulse is set to, for example, 170V.
The following will describe the sustaining period 6 specifically. In the sustaining period 6, first, a positive sustaining pulse (hereinafter called first sustaining pulse) is applied to the scanning electrode S and the ground potential is applied to the common electrode C. Note here that in the sustaining period 6, the potential of the data electrode D is always at the ground potential. Then, in a display cell where write-in discharge has occurred in the scanning period 5, a large positive charge is formed over the scanning electrode S and a large negative wall charge is formed over the common electrode C, so that a wall voltage due to this positive wall charge is superimposed on the first sustaining pulse applied to the scanning electrode S to thereby apply a voltage higher than a surface-firing voltage to the inter-face gap, thus generating sustaining discharge. The sustaining discharge thus generated causes a negative wall charge to build up over the scanning electrode S and a positive wall charge to build up over the common electrode C. In a display cell where write-in discharge has not occurred in the scanning period 5, no wall voltage is superimposed on the first sustaining pulse and so a voltage of the inter-face gap does not reach the surface-firing voltage, so that sustaining discharge does not occur.
A next sustaining pulse (hereinafter called second sustaining pulse) is applied to the common electrode C. At the same time, the ground potential is applied to the scanning electrode S. In this case, in a display cell where sustaining discharge has occurred owing to the above-mentioned first sustaining pulse, the second sustaining pulse is superimposed on a wall charge formed through sustaining discharge due to this first sustaining pulse, thus generating sustaining discharge. Accordingly, a wall charge having a polarity opposite to that when sustaining discharge has occurred owing to the first sustaining pulse builds up over the scanning electrode S and the common electrode C. That is, a wall charge arrangement returns to that shown in FIG. 10E. From this moment on, discharge occurs sustainedly based on almost the same principle. That is, a potential difference due to a wall charge generated by the x'th sustaining discharge is superimposed on the (x+1)'th sustaining pulse to thereby sustain sustaining discharge. The number of times this sustaining discharge is sustained determines an amount of light emitted.
At a picture cell where no write-in discharge has occurred, on the other hand, no wall charge is superimposed on a sustaining pulse. Application of the sustaining pulse alone is not enough to attain a discharge starting voltage, so that surface discharge does not occur.
The above-mentioned preliminary period 7, scanning period 5, and sustaining period 6 are combined to make up the sub-field 8. To display an image on a PDP, gradation of the image can be displayed by providing mutually different numbers of sustaining pulses in different sub-fields in one field which is a period for displaying image information of one screen and selecting whether each of these sub-fields is to be lit up or not in order to control the number of times of generating sustaining discharge.
This conventional technology, however, has the following problems. First, in a conventional PDP, as shown in FIG. 10D, immediately preceding the scanning period 5, such a wall charge arrangement is present that a negative wall charge is formed over the scanning electrode S and a positive wall charge is formed over the common electrode C. Therefore, if write-in discharge occurs in the scanning period 5, as described above, charged particles generated by discharge at the opposed gap spread in a display cell and so move also between respective surface electrodes of the scanning electrode and the common electrode. Accordingly, a larger write-in discharge current occurs to increase power dissipation of a scanning driver and costs thereof as well.
Second, as shown in FIGS. 10B-10D, surface discharge occurs in the priming period 3 and the priming erasing period 4, so that black luminance of the PDP becomes large to decrease contrast of image display.